1. Field of Invention
The present invention relates to a light guide plate, a surface light source device and a display, in particular, a light guide plate having a corner portion from which light is inputted, a surface light source device employing the light guide plate and a display having a liquid crystal display panel illuminated by the surface light source device. The invention is applied, for example, to a liquid crystal display incorporated in a portable phone, and to a surface light source device and light guide plate employed in the display.
2. Related Arts
It is well known to employ a surface light source device to illuminate a liquid crystal display panel in a liquid crystal display. In general, a surface light source device comprises a light guide plate and a primary light source to supply primary light to the plate. Although rod-like fluorescent lamps (cold cathode tubes) have been broadly adopted as primary light sources, a recent tendency is that alternative ones using a point-like light emitter such as light emitting diode (called LED hereafter) are growingly employed.
In particular, the above tendency is being predominant in cases where a liquid crystal display has a comparatively narrow display screen incorporated in a comparatively small appliance such as portable phone.
With such a surface light source device, light is introduced into a light guide plate, then being emitted gradually from an emission face on the way of propagation within the plate. That is, such an inside propagation gives light chances of inner incidence to the emission face, with the result that some of the inner-incident light overcomes the critical angle condition to escape out of the light guide plate through the emission face, hereby providing an emission output. But some problems arise as follows.
(1) A very small part of the light can really overcome the critical angle condition. Accordingly, a low emission efficiency is realized.
(2) As well known, a main direction of emission is greatly inclined with respect to a frontal direction of an emission face. This makes it difficult to supply an object to be illuminated, such as liquid crystal display panel, with light from a generally vertical direction.
(3) An point-like light emitter employed, the light emitter brings a divergent property in addition to so-called xe2x80x9coblique emission directionalityxe2x80x9d as above (2), resulting in an inefficient illumination. It hardly gives a display, if applied to the display, a clear display screen.
According to a known technique for solving such problems, a light scattering pattern is formed on an emission face. In some cases, a light scattering pattern is formed on a back face. Such a light scattering pattern, which consists of a great number of fine light scattering elements formed according to a predetermined distribution, promotes emission from an emission face to relax above problem (1). In addition to this, a light scattering effect reduces the directionality of emission, thereby relaxing above problems (2) and (3).
However, such a technique relying on a light scattering pattern is not capable of directing an emission positively to a desired direction (usually, a roughly frontal direction), although it is able to weaken the directionality through a light scattering effect.
Although other prior arts include a method, according to which an emission face or back face is roughened overall, and another method, according to which a light diffusion sheet is arranged additionally along an emission face, such methods are also hardly expected to be capable of directing an emission positively to a desired direction (usually, a roughly frontal direction) because they depend on causing propagation directions of emission to be diversified in order to overcome the problems.
Although a well known prism sheet has a function of directing an emission positively to a desired direction, such an additional sheet, if employed and disposed, will lead to an increased number of components and accordingly to an increased manufacturing cost. Beside, if applied, in particular, to a small size appliance such as portable phone, it brings a disadvantage because a total thickness of the appliance will be increased.
The present inventor proposed a technique according to which a great number of projection-shaped micro-reflectors are formed on a back face of a light guide plate to overcome the above-described situation (U.S. patent application Ser. Nos. 09/647,760; WO 00/49432).
The propose is briefly describe with reference to FIGS. 14 and 15. FIG. 14 shows one of projections which are formed on a back face of light guide plate. FIG. 15 exemplarily shows a distribution of the projections on the back face of the light guide plate and an arrangement of a primary light source.
As illustrated in FIG. 14, each of the projections is shaped as to project from a general plane of the back face (i.e. a plane representative of the back face except for the projections). Each projection comprises a first reflection face (slope) and second reflection face (slope), a pair of these reflection faces forming a valley inside.
An inside propagating light entering into a valley formed by any projection, most of the light is inner-reflected by one and the other of a pair of the reflection faces in succession to be directed toward an emission face. Since an inner-incidence angel to the emission face can be controlled through directions of the two reflection faces, an emission to, for example, a generally frontal direction is realized with ease. This function of micro-reflectors depends on no light diverging effect (neither scattering nor diffusing). There is no need to arrange a prism sheet.
And, it is allowed to adopt a line-like light source such as cold cathode tube or point-like one such as LED optionally as a primary light source. An arrangement in which the latter is adopted is illustrated in FIG. 15. Referring to the arrangement, LEDs are disposed at two positions along one side of a light guide plate and projections are laid out correlatively with the LEDs.
Viewing from small-sizing of device or saving of electric power, it is preferable to adopt a LED as a primary light source. And controlling of light supply quantity is easily achieved by, for example, employing a drive circuit designed as to be capable of optional lighting. It will be growingly expected and desired in the future that the arrangement as shown in FIG. 15 is applied to displays for various portable information appliances such as portable phones.
Under such a situation, the present inventor has been studying on arrangements employing a plurality of LEDs as shown in FIG. 15 and has come to understandings as follows.
An arrangement as shown in FIG. 15 suffers from a problem that an projection array determined as to be optimum for one LED is far from being optimum for the other LED because a plurality of LEDs are disposed along a side away from each other. In other words, it is hardly possible to determine an projection array as to be optimum for every LED for direction-converting light emitted from every LED effectively to a desired direction effectively.
Another problem, which can arise in some cases, is that projections fitting for one LED can disturb an inside propagation or direction conversion of light emitted from the other LED. Such a situation apparently leads to a conclusion that arrangements like one shown in FIG. 15 should be improved, viewing from an expectation that demand for a high efficiency of light utilization and a uniform brightness on an emission face will be more severe in the near future.
That is, if a plurality of point-like light emitters are combined with a light guide plate provided with micro-reflectors as above-proposed, a newly structured surface light source device and a novel light guide plate employed in the device fitting thereto are desired.
An object of the present invention is to provide a light guide plate improved as to satisfy the above-mentioned demand, a surface light source device improved by making use of the light guide plate and a display improved by making use of the surface light source device.
Saying concretely, the present invention provides a light guide plate comprising a micro-reflector disposal which can be fit for a plurality of point-like light emitters, a surface light source device which employs the light guide plate suitably in combination with point-like light emitters and a display which employs the surface light source device for illuminating a liquid crystal display panel.
First, the present invention is applied to a light guide plate to be supplied with light from a primary light source having a point-like light emitter, wherein the light guide plate comprises an emission face for light output, a back face opposite with the emission face and an incidence face for light input.
According to the present invention, the incidence face is located at a corner portion of the light guide plate and the back face is provided with a great number of projections for light-advancing-direction-conversion.
Each of the projections includes a pair of slopes formed so as to provide a ridge approaching the back face with an increasing distance from the incidence face. And pairs of slopes provide valleys in the light guide plate, the valleys being orientated to be opened toward the corner portion. In a typical case, the projections are configurated like quadrangle pyramids.
Such a construction is suitable for a light guide plate having an incidence face supplied with light from a plurality of point-like light emitters. That is, if a plurality of point-like light emitters are combined with the light guide plate, no great difference arises among light supply paths of the respective point-like light emitters because the incidence face is located at a corner portion. This contrasts apparently with the arrangement shown in FIG. 15. In the case of the arrangement shown in FIG. 15, the incidence faces located at two locations distant from each other cause the corresponding LEDs to produce light paths which are greatly different from each other. The present invention is able to avoids such a situation.
Accordingly, the micro-reflectors, which are orientated as to be opened toward the corner portion as mentioned above, a plurality of point-like light emits supply light which is effectively direction-converted by the micro-reflectors, thereby realizing an emission positively directed to a desired direction.
In the next place, the present invention is applied to a surface light source device comprising a primary light source having a point-like light emitter and a light guide plate supplied with light from the primary light source, the light being introduced into the light guide plate through an incidence face and being emitted through an emission face.
According to the present invention, the incidence face is located at a corner portion of the light guide plate and the back face is provided with a great number of projections for light-advancing-direction-conversion.
Each of the projections includes a pair of slopes formed so as to provide a ridge approaching the back face with an increasing distance from the incidence face. And pairs of slopes provide valleys in the light guide plate, the valleys being orientated to be opened toward the corner portion. In a typical case, the projections are configurated like quadrangle pyramids.
Such a construction causes the disposed plurality of point-like light emitters to supply light which is introduced into the light guide plate through the incidence face located at the corner along light paths similar to each other and then is direction-converted effectively by the micro-reflectors, thereby realizing an emission positively directed to a desired direction. It is noted that directions toward which valleys of the micro-reflectors are opened only need to be directed substantially to the corner.
That is, the respective valleys orientated as to be opened toward the corner portion cause the plurality of point-like light emitters disposed to produce light which approach the micro-reflectors from the corner portion and is guided to the valleys very smoothly no matter which emitter is origin of the guided light. Needless to say, such an advantage is not expected in the arrangement as shown in FIG. 15.
A display in accordance with the present invention is obtained by disposing the surface light source device improved as above for illuminating a liquid crystal display panel. The display inherits the above advantage of the surface light source device. That is, the liquid crystal display panel is illuminated effectively from a desired direction, providing a clear display screen.